This invention relates to a dental handpiece for supporting a rotatable dental burr.
A dental handpiece comprises an elongated arm to be grasped by the dentist, and a hollow head formed at the distal end of the arm. A holder of a generally cylindrical shape is rotatably supported on the head through a pair of bearings. A shank of the dental burr is fitted in and fixed to the holder, with a grinding portion of the dental burr projected exteriorly of the head. The holder is rotated by compressed air, so that the dental burr held by the holder is rotated.
When the rotating dental burr grinds a tooth or the like, a grinding heat is produced. Therefore, it is necessary to supply cooling water (coolant) to the dental burr and the tooth so as to prevent the overheating of them. Generally, the cooling water is injected from a hole formed in that portion of the arm disposed near the head.
The following piror art disclosed dental handpieces in which cooling water is injected from a distal end portion of a dental burr so as to enhance the cooling effect.
Japanese Laid-Open (Kokai) Patent Application Nos. 99550/89 and 110358/89 (Based on these two applications and other application, U.S. Patent Application was filed Dec. 21, 1988 under Ser. No. 287,116, now U.S. Pat. No. 5,022,857) and U.S. Pat. No. 3,762,052 disclose dental burrs which have an axially-extending auxiliary coolant passageway. These prior art publications also disclose dental handpieces in which a nozzle is formed on an inner surface of a head in coaxial relation to a holder and the dental burr. An arm of the dental handpiece has a main coolant passageway extending in the longitudinal direction thereof. The proximal end of the main coolant passageway is connected to a cooling water source. The main coolant passageway extends to the head, and is connected at its distal end to the above-mentioned nozzle. The nozzle is inserted in an opening formed in one end portion of the holder disposed close to the nozzle. The cooling water of the cooling water source is injected from the nozzle via the main coolant passageway of the handpiece. The cooling water injected from the nozzle passes through the auxiliary coolant passageway of the dental burr, and is injected from the distal end of the dental burr.
The nozzle is stationary whereas the holder is rotating at high speed (for example, 300,000 r.p.m.), and therefore the outer peripheral surface of the nozzle must be kept out of contact with the inner peripheral surface of the end portion of the holder surrounding the nozzle, so that a gap is formed therebetween. Therefore, the cooling water injected from the nozzle leaks through this gap into the internal space of the head to reach one of bearings disposed close to the nozzle, thereby rusting this bearing. As a result, the rotation of the holder is affected. Also, the pressure of injection of the cooling water from the dental burr can not be kept to a sufficient level.
The above Japanese Laid-Open Patent Application No. 99550/89 discloses pump means which draws the cooling water, residing in the gap between the outer peripheral surface of the nozzle and the inner peripheral surface of the end portion of the holder surrounding the nozzle, toward the auxiliary coolant passageway of the dental burr. This pump means includes a spiral projection formed on the inner peripheral surface of the end portion of the holder. When the holder rotates, the spiral projection also rotates, and the cooling water is drawn by the rotation of the spiral projection. However, with this pump means, a suction (drawing) effect of a high level could not be obtained. The reason for this is through to be as follows. In order for the spiral projection to effect the pumping action, the spiral projection must rotate relative to the cooling water residing in the gap between the nozzle and the end portion of the holder. In other words, it is necessary that this cooling water should be stationary or be rotating at a speed lower than that of the spiral projection. Actually, however, when the holder rotates, the cooling water also rotates therewith because of the friction, and therefore the difference between the rotational speed of the cooling water and the rotational speed of the spiral projection is small, so that a high pumping action can not be obtained.